In the refinery and petrochemical industry, the most commonly used materials for reactors and heat exchangers are carbon steels due to cost consideration. Often, high alloy steels are used only for hydrocarbon processes where other requirements such as corrosion or operating temperature may become an issue. It is well-known that iron and its oxides present on steel surfaces could act as promoters for coke formation.
Coke formation on equipment surfaces could cause many problems for process operation. Among them, two often mentioned problems are the reduced (distorted) heat transfer across the equipment walls due to the build-up of coke deposits having poor thermal conductivity, and increased pressure drop due to the accumulated coke deposit which can substantially reduce the opening for the process stream and which also increases the surface roughness in contact with hydrocarbon stream. Both of these effects can affect the designed performance of a particular equipment. Other problems with coke formation in hydrocarbon processing equipment include loss of operation time and the required maintenance cost for coke removal using on-line or off-line methods. For example, in transfer line exchangers used for quenching the effluent stream from a steam cracker, coke formation often becomes a major problem restricting furnace run length, especially for naphtha cracking. With emerging technologies for longer furnace run length, coke formation in the transfer line exchangers must be dealt with.
There have been a number of proposals for treatment of steels to reduce their tendency to coke when exposed to hydrocarbons at elevated temperatures. In general, these proposals in the prior art could fall into two categories—the use of coke inhibiting compounds or mixtures to react with the steel surface and form an inert surface prior to its exposure to process hydrocarbons and/or during hydrocarbon processing, and surface passivation through treatment using gases such as hydrogen, carbon dioxides, air or steam prior to exposure to hydrocarbons.
Injection of coke inhibiting compounds or mixtures has become a very popular approach for technology development and to some extent for plant practice.
U.S. Patent Application 20020029514 published Mar. 14, 2002 assigned to Atofina Chemicals Inc. teaches treating a furnace, preferably co-injecting with steam and one or more compounds of the formula R—Sx—R′ where x is an integer from 1 to 5 and R and R′ are selected from the group consisting of a hydrogen atom and a C1-24 straight chain or branched aryl radicals, and one or more compounds of the formula:
wherein R, R′ and R″ are selected from the group consisting of C1-24 straight or branched aryl radicals. The present invention has not only eliminated the hydroxylamines, hydrazines and amine oxides required by the prior art, but also identified additional but essential steps to make the passivation of steel surface more stable.
U.S. Pat. No. 4,636,297 issued Jan. 13, 1987 to Uchiyama et al., assigned to Hakuto Chemical Co., Ltd. teaches applying a mixture of dialkyl thioureas and thiuram mono- and/or di-sulfides in an amount from 10 to 5,000 ppm to the surface of a reactor prone to coke formation. The reference does not teach the specific components used in the present invention nor does it disclose the preliminary reduction nor the curing steps required in the present invention.
U.S. Pat. No. 5,777,188 issued Jul. 7, 1998 to Reed et al., assigned to Phillips Petroleum Company discloses adding to the feed of a steam cracker with steam as a carrier gas and a mixture of polysufides of the formula R—Sx—R′ wherein R and R′ are independent hydrocarbyl radical having 1 to about 30 carbon atoms and x is a number from about 3 to 10. The proposed weight ratio of polysulfides to steam is in the range from about 0.00002:1 to about 1:1. Again the reference fails to teach the specific components used in the present invention nor does it disclose the preliminary reduction and the curing steps required in the present invention.
In addition, there are many other chemicals or mixtures of them that could be used for reduction of coke formation under cracking and TLE operating conditions. Tong et al. has claimed a number of organic phosphorous compounds (U.S. Pat. Nos. 5,354,450; 5,779,881; 5,360,531 and 5,954,943, assigned to Nalco/Exxon) that can be used as coke inhibitors for coke reduction under coil and TLE conditions. A combination of gallium, tin, silicon, antimony, and aluminum has also been claimed in the prior art (U.S. Pat. Nos. 4,687,567; U.S. 4,692,234; and U.S. 4,804,487), assigned to Phillips Petroleum. Additionally, certain inorganic salts, a mixture of Group IA and IIA metal salts and a boron acid (U.S. Pat. No. 5,358,626) assigned to Tetra International, have been claimed as effective in coke reduction under coil conditions. Once again, these references fail to teach the specific components used in the present invention nor do they disclose the preliminary reduction nor the curing steps required in the present invention.
The other group of methods or processes available in the prior art, teaches the use of gases, such as H2, carbon oxides, steam and air to treat steel surfaces prior to their exposure to hydrocarbon process streams in order to minimize the coking propensity of steel surfaces.
U.S. Pat. No. 5,501,878 issued Mar. 26, 1996, assigned to Mannesmann Aktiengesellschaft; KTI Group B.V. teaches treating the surface of a heat exchanger which comes in contact with hydrocarbons with a mixture of steam and 5 to 20 weight % hydrogen at a temperature from about 400° C. to 550° C. for from 1 to 6 hours to reduce Fe2O3, that is catalytically active to produce coke, to Fe3O4 that is not as active to produce coke. The present invention uses a lower amount of hydrogen than that specified in the reference and comprises further steps not disclosed in the reference.
U.S. Pat. No. 6,436,202 issued Aug. 20, 2002, assigned to NOVA Chemicals teaches a process for treating stainless steel comprising from 13–50 weight % Cr, 20–50 weight % Ni and at least 0.2 weight % Mn in the presence of a low oxidizing atmosphere, which comprises from 0.5 to 1.5 weight % of steam, from 10 to 99.5 weight % of one or more gases selected from the group consisting of hydrogen, CO and CO2 and from 0 to 88 weight % of an inert gas selected from the group consisting nitrogen, argon and helium. In an earlier U.S. Pat. No. 5,630,887, again assigned to NOVA Chemicals (previously NOVACOR Chemicals) a similar procedure was proposed for the treatment of stainless steel furnace tubes which are used in the petrochemical industry. This treatment involves exposing stainless steel to an atmosphere containing a low amount of oxygen at temperatures up to 1200° C. for up to about 50 hours. The stainless steel treated according to such a procedure will have a lower tendency to coke formation during use. However, these treatments are not suggested for steels with a Cr content less than 13 weight %, for instance, carbon steel, which comprises typically less than 5 weight % Cr. In addition, the required use of the coke inhibiting compounds of the present invention and the curing step have not been disclosed in these references.
The present invention seeks to provide an effective method of treating a steel, preferably but not limited to carbon steels, subject to conditions where coke is likely to form to reduce coke formation.